1. Field of the Invention
This invention relates to printing plates which are electrostatically imaged, to methods of preparing such printing plates, and to methods of printing using such printing plates. More particularly, the printing plates of this invention are prepared by applying an alkali soluble coating composition comprising at least one polymer composition to a hydrophilic surface on a substrate to provide the surface with at least one alkali soluble layer. The coated substrate is electrostatically imaged using a toner composition which is applied to the alkali soluble layer. The imaged substrate is heated a first time to fuse the toner composition to the alkali soluble layer, thereby protecting the underlying alkali soluble layer from subsequent contacting with developer solution in the imaged areas. The imaged plate is thereafter contacted with an aqueous alkali solution to remove undesired toner composition and the non-imaged portion of the alkali soluble layer which is unprotected by the fused toner composition, and the imaged plate is thereafter heated a second time to fix the remaining toner and underlying alkali soluble layer to the substrate.
2. Background Information
The manufacture of printing plates, including printing plates used in lithographic printing processes, using electrostatic imaging techniques is well known in the art. In such methods, the fixed toner images are the olephilic ink receptive portions of the plate, and upon contact of the plate with an appropriate ink or ink-containing solution, the desired ink image may be transferred, or xe2x80x9coffset,xe2x80x9d from the plate to an appropriate medium, such as a rubber blanket, which is then used to print onto a medium such as paper. Examples of methods of preparing printing plates which are electrostatically imaged include:
U.S. Pat. No. 3,315,600, which discloses a method for preparing a printing plate in which a support having a hydrophilic surface is provided with a covering layer, the covering layer is electrostatically imaged using a toner composition, the image is fused or fixed via heating, and the covering layer is removed from the non-imaged areas by means of an aqueous solvent. However, unlike the invention described herein, only a single heating step is employed to fix the toner image to the coated support.
U.S. Pat. No. 4,444,858, which discloses a method of preparing a lithographic printing plate in which a metal substrate is coated with a synthetic resin layer, and a toner image formed on a photosensitive sheet by an electrophotographic process is transferred and fixed to the synthetic resin layer. A solvent is used to remove the non-imaged areas of the resin layer, which are not covered by the fixed toner image. However, unlike the invention described herein, the remaining toner layer which makes up the image area is used as a mask and thereafter removed to expose the insoluble imaged portion of the synthetic resin layer. In addition, unlike the present invention, no second heating or fusing step is disclosed.
U.S. Pat. No. 4,457,992, which discloses an etchable electrophotographic printing plate comprising an electroconductive support coated with a light-sensitive photoconductive zinc oxide and a sensitizing dye dispersed in an organic resin binder. Such plates are typically referred to as xe2x80x9corganic photoconductorxe2x80x9d or xe2x80x9cOPCxe2x80x9d plates. The coating is applied to the substrate as a thin film (i.e. about 5-50 xcexcm), and dried to remove substantially all of the solvent. The resulting plate may be imaged with electrostatic toner, and the non-imaged portions of the coating are removed via a basic aqueous solution. The plate may thereafter optionally be heated to enhance plate endurance. However, unlike the invention described herein, the coating requires light-sensitive photoconductive zinc oxide to be used. In contrast, in the present invention, no light-sensitive photoconductive coating is applied to the hydrophilic surface.
U.S. Pat. No. 6,025,100, which discloses a printing plate prepared by transferring a toner image to an image receiving element which is a support having an image receiving layer thereon. The layer contains a hydrophilic binder, TiO2 particles, and a matting agent, and the layer is cross-linked with hydrolyzed tetramethyl silicate or hydrolyzed tetraethylsilicate. However, unlike the invention described herein, there is no disclosure of a second heating or fusing of the toner to the imaged receiving element to fix the remaining toner and underlying alkali soluble layer in the imaged areas.
Lithographic printing plates having an imageable layer overlaid upon an intermediate layer applied to a substrate are also known. For example, U.S. Pat. No. 6,014,929 discloses a lithographic plate having a rough substrate, a releasable interlayer applied to the rough substrate surface, and a radiation-sensitive layer applied to the interlayer. However, unlike the invention described herein, there is no disclosure of the use of two separate heating or fusing steps with electrostatic imaging.
However, electrostatically imaged printing plates are known to have several disadvantages. For example, such plates are typically limited to uses for low quality printing and single color printing, due to limitations associated with imaging resolution, image width, imaging speed and unwanted background toning in non-imaged areas of the plate. Unwanted background toning on the plate substrate due to uncontrolled application of toner causes the printing of an undesirable toned background on press which limits the quality and utility of electrostatic imaging of printing plates. This unwanted phenomenon is commonly referred to as xe2x80x9cspeckingxe2x80x9d due to the formation of undesired specks in the background of the printed medium.
Several attempts have been made in the art to overcome the problem of unwanted background toning or specking. For example, the use of OPC plates with liquid toners has been attempted, but such systems are typically capable of only low-resolution imaging, and in addition are expensive and have environmental restrictions. Cleaning solutions for flexible base plates have also been employed to scrub the plate background area and remove unwanted background toner, but this method is highly operator-dependent and often results in damage to the imaged area of the plate.
In view of the foregoing, it would be advantageous to employ electrostatic imaging of a printing plate in such a manner as to minimize or eliminate undesired background toning, and simultaneously avoid the use of OPC plates and cleaning solutions. It is one object of this invention to provide an electrostatically imaged printing plate which is capable of high quality and multi-color printing. It is another object of this invention to provide a method of preparing such a printing plate. It is yet another object of this invention to provide a method of printing using such a printing plate. The printing plate of this invention advantageously avoids the above-described disadvantages of OPC plates and the use of cleaning solutions. The printing plate of this invention also advantageously may be employed for high quality printing applications such as four-color newspaper printing, book printing, financial and other small format printing, and the like, where such quality printing requires the minimization or elimination of undesired background toning and unwanted specking on the printed medium. The printing plate of this invention is also capable of long print run lengths (i.e. run lengths greater than say 150,000).
A printing plate is prepared by the process comprising:
(a) applying an alkali soluble composition comprising at least one polymer composition to a hydrophilic surface on a substrate to provide the surface with at least one layer which is alkali soluble at a pH in the range of about 6.0 to 14.0;
(b) imaging the coated substrate electrostatically with a toner composition;
(c) heating the imaged substrate a first time to a temperature greater than the glass transition temperature of the toner composition;
(d) contacting the imaged substrate with an aqueous alkaline solution having a pH in the range of about 6.0 to about 14.0; and
(e) heating the imaged substrate a second time to a temperature greater than the glass transition temperature of the toner composition.
In a preferred embodiment, the substrate is an aluminum substrate having at least one hydrophilic surface which receives the coating composition. The polymer composition may be an acrylic resin. The coating may additionally comprise a cross-linking composition, such as a titanium complex. The total coating weight is in the range of 0.2-3 g/m2, more preferably 0.5-3 g/m2. The electrostatically applied and fused toner is preferably insoluble in an alkali solution in comparison to the polymer composition employed in this invention. The printing plate of this invention is capable of long runs on press with good image quality and advantageously minimizes the occurrence of undesirable background toner xe2x80x9cspeckingxe2x80x9d on press. Accordingly, the printing plate is satisfactory for demanding printing tasks such as four-color newspaper printing, financial printing and the like.